3d microscope including insertable components to provide multiple imaging and measurement capabilities

ABSTRACT

A three-dimensional (3D) microscope includes various insertable components that facilitate multiple imaging and measurement capabilities. These capabilities include Nomarski imaging, polarized light imaging, quantitative differential interference contrast (q-DIC) imaging, motorized polarized light imaging, phase-shifting interferometry (PSI), and vertical-scanning interferometry (VSI).

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 16/056,860entitled “3D Microscope Including Insertable Components To ProvideMultiple Imaging And Measurement Capabilities”, filed Aug. 7, 2018,which is a divisional of U.S. application Ser. No. 13/333,938 entitled“3D Microscope Including Insertable Components To Provide MultipleImaging And Measurement Capabilities” and filed Dec. 21, 2011, now U.S.Pat. No. 10,048,480, which claims priority to U.S. ProvisionalApplication 61/430,937, entitled “3D Imaging and Metrology System” andfiled on Jan. 7, 2011.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to optical imaging and measurement systems andmore particularly to a three-dimensional (3D) microscope includinginsertable components that facilitate multiple imaging and measurementcapabilities, including Nomarski imaging, polarized light imaging,quantitative differential interference contrast (q-DIC) imaging,motorized polarized light imaging, phase-shifting interferometry (PSI),and vertical-scanning interferometry (VSI).

Related Art

Conventional microscopes enable an operator to view magnified images ofminute features on samples otherwise invisible to the human eye. Becauseof this, conventional microscopes have been widely used in universities,research institutes, and many industries. However, a conventionalmicroscope has a significant limitation. Specifically, a conventionalmicroscope only provides a two-dimensional (2D) image of a sample, whilein the real world a majority of samples are 3D in nature. Therefore aneed arises for a 3D microscope to capture images of those samples.

SUMMARY OF THE INVENTION

A microscope illuminator capable of generating 3-D images is provided.This microscope illuminator includes a first light source forming partof a first light path, and a second light source and a set of articlesforming part of a second light path. The set of articles can include aplurality of patterned articles, and one of a through-hole and apin-hole. The first and second light paths can have a shared set ofcomponents, which can include a first beam-splitter, a lens group, and abeam-splitter set.

The first light source can direct light onto the first beam-splitter andthe second light source can direct light via one of the set of articlesonto said first beam-splitter. A surface of the one article can belocated at an effective focal plane of said lens group. The lens groupcan image the first light source and the second light source at anentrance pupil of a microscope objective lens via the beam-splitter set.The beam-splitter set can include a second beam-splitter and a pair ofbeam-splitters mounted on a first linear slider linked to a first pulllever.

The microscope illuminator can further include a focusing lens and amulti-pin connector. The beam-splitter set and the focusing lens canform part of a third light path for directing light to an externalconnector. The multi-pin connector can link electronically to the firstand second light sources.

The set of articles can be mounted on a second linear slider linked to asecond pull lever. Each patterned article can be transparent materialwith a pattern formed on one of its surfaces. In one embodiment, thefirst and second light sources are light emitting diodes (LEDs).

A housing for the microscope illuminator can include a slot forinserting one of a plurality of components, wherein when inserted, eachcomponent is positioned to form part of the first and the second lightpaths. One of the components can be a polarizer assembly including anadjustable polarizer. For Nomarski imaging, this polarizer is set at afixed orientation, and the second linear slider is positioned such thatone of the patterned articles or the through-hole is in the first lightpath. For polarized light imaging applications, an orientation of thepolarizer is adjustable, and the second linear slider is positioned suchthat one of the patterned articles or the through-hole is in the secondlight path.

Another of the components can be an assembly including a polarizer witha motorized rotator and a quarter wave plate, wherein the motorizedrotator is connected to the multi-pin connector. The motorized rotatorcan be controlled remotely by a recipe, the recipe being based onobservation type and particular sample imaging. For quantitativedifferential interference contrast (q-DIC), the polarizer is steppedthrough a predetermined number of consecutive steps having apredetermined phase shift. In one embodiment, another of the componentscan be a wavelength filter assembly including a through-hole and anarrow band filter.

A 3D microscope is also provided. This 3D microscope can include anilluminator capable of generating 3D images of a sample, the illuminatorincluding a first slot for a first component. A turret can be mounted onthe illuminator, wherein the turret can include a second slot for secondcomponent. An objective lens can be mounted on the turret. A tube lensand adaptor can be mounted on the illuminator, wherein the adaptor caninclude a third slot for third components. An optical sensor and opticalsensor coupler can be mounted on the tube lens and adaptor, wherein theoptical sensor can be configured to acquire images of the sample. Aprocessor is included for controlling the illuminator and the opticalsensor, wherein the first, second, and third components facilitateNomarski imaging, and the first and third components facilitatepolarized light imaging.

The optical sensor can include one of a charge-coupled device (CCD)camera and a complementary metal-oxide semiconductor (CMOS) camera. Theoptical sensor coupler can provide a plurality of magnifications for theoptical sensor. A spectrometer can be coupled to the illuminator,wherein light for the spectrometer is collected via a path independentof an imaging path leading to the optical sensor. The 3D microscope canalso include a focusing adjustment device that provides a plurality of Zstep adjustments to the sample. In one embodiment, the focusingadjustment device can be mounted on one of a sample chuck and theturret.

The objective lens can include a Michelson interference objective lensand/or a Mirau interference objective lens mounted on the turret. Forvertical-scanning interferometry (VSI), the first component can includea filter assembly positioned with a through-hole in an illuminationlight path, and the positioning means can be configured to move thesample in the Z direction while the optical sensor capturesinterferograms, thereby creating a true-color 3D image of the sample.For phase-shifting interferometry (PSI), the first component can includea filter assembly positioned with a filter in an illumination lightpath, and the positioning means can be configured to make four phaseshift moves while the optical sensor captures four interferograms. Theturret, when rotated to operate without the objective lens, cantransform the 3D microscope into an autocollimator.

A method of 3D imaging or measuring a patterned substrate sample is alsoprovided. The method can include varying a relative distance between thepatterned substrate sample and an objective lens at predetermined steps.At first predetermined steps, an image of a patterned article can beprojected onto a focal plane of the objective lens. A first image with apattern associated with the patterned article and the sample can becaptured and then stored in a first image array. At second predeterminedsteps, wherein the second predetermined steps have a different number ofsteps than the first predetermined steps, a second image of the samplewithout the pattern associated with the patterned article can becaptured and then stored in a second image array. The first and secondimages can be analyzed to 3D image or measure the patterned substratesample.

The number of second predetermined steps can be less than that of thefirst predetermined steps. The first and second predetermined steps canbe allocated to specific levels. The first and second predeterminedsteps can skip predetermined levels of the sample. At least one of thefirst and second predetermined steps can have uneven steps.

Another method of 3D imaging or measuring a patterned substrate sampleis provided. In this method, a relative distance between the patternedsubstrate sample and an objective lens can be varied at predeterminedsteps. At first predetermined steps, an image of a patterned article canbe projected onto a focal plane of the objective lens. A first imagewith a pattern associated with the patterned article and the sample canbe captured and then stored in a first image array. At secondpredetermined steps, wherein the second predetermined steps have adifferent number of steps than the first predetermined steps, a secondimage of the sample without the pattern associated with the patternedarticle can be captured and then stored in a second image array. Thefirst and second images can be analyzed to 3D image or measure thepatterned substrate sample. The method can further include performing adownward scan and an upward scan to determining drooping effects, andthen providing resulting step height values when analyzing the first andsecond images.

A method of repositioning a sample to minimize tilt is also provided. Inthis method, a light source of a 3D microscope can be turned on, whereinthe light source passes through a pin-hole, and the 3D microscope istransformed into an autocollimator. When an image of the pin-hole fallsinside a field of view, a tip/tilt mechanism of a stage of the 3Dmicroscope can be adjusted so that the pin-hole image coincides with apre-defined circle on an otherwise pitch dark field of view, therebycompleting alignment. When the image of the pin-hole falls outside thefield of view, a one-pass 3D imaging acquisition process and adjustmentof the stage can be performed based on the process to bring the image ofthe pin-hole into the field of view. Then, the tip/tilt mechanism of thestage of the 3D microscope can be adjusted so that the pin-hole imagecoincides with the pre-defined circle on the otherwise pitch dark fieldof view, thereby completing alignment.

Another method of repositioning a sample to minimize tilt is alsoprovided. In this method, a light source of a 3D microscope can beturned on, wherein the light source passes through a pin-hole, and the3D microscope can be transformed into an autocollimator. When an imageof the pin-hole falls inside a field of view, a tip/tilt mechanism of astage of the 3D microscope can be adjusted so that the pin-hole imagecoincides with a pre-defined circle on an otherwise pitch dark field ofview, thereby completing alignment. When the image of the pin-hole fallsoutside the field of view, the tip/tilt mechanism can be coarse adjustedwhile watching for the pin-hole image to appear within the field ofview. Then, the tip/tilt mechanism of the stage of the 3D microscope canbe fined adjusted so that the pin-hole image coincides with thepre-defined circle on the otherwise pitch dark field of view, therebycompleting alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an exemplary illuminator used in a 3D imaging andmetrology system.

FIG. 1B illustrates an exemplary patterned article for use in a 3Dimaging and metrology system.

FIG. 1C illustrates an exemplary illuminator configured for Nomarski andpolarized light imaging.

FIG. 1D illustrates an exemplary illuminator configured for quantitativedifferential interference contrast (q-DIC) and motorized polarized lightimaging.

FIG. 1E illustrates an exemplary illuminator configured forphase-shifting interferometry (PSI) and vertical-scanning interferometry(VSI).

FIG. 2A illustrates an exemplary 3D imaging and metrology system, whichincludes an illuminator configured for film thickness measurement.

FIG. 2B illustrates another exemplary 3D imaging and metrology system,which includes an illuminator configured for Nomarski and polarizedlight imaging.

FIG. 2C illustrates another exemplary 3D imaging and metrology system,which includes an illuminator configured for q-DIC and motorizedpolarized light imaging.

FIG. 2D illustrates another exemplary 3D imaging and metrology system,which includes an illuminator configured for phase-shifting and verticalscanning interferometry.

FIG. 3A illustrates an exemplary tip/tilt alignment technique.

FIG. 3B illustrates exemplary surfaces of a sample and potentialtargeted levels of user interest.

FIG. 4 shows a limited q-DIC image of an II-VI compound semiconductorwafer surface acquired on an implemented system conforming to that ofFIG. 2B.

FIGS. 5A and 5B illustrate other exemplary 3D imaging and metrologysystems including motorized tip and tilt stages.

FIG. 6 illustrates an exemplary one-pass 3D image acquisition process.

DETAILED DESCRIPTION OF THE DRAWINGS

State of the art 3D imaging and measurement techniques are set forth incommonly assigned U.S. Pat. Nos. 7,729,049 and 7,944,609 as well asco-pending U.S. Published Applications 20100135573 and 20080291533, allof which are incorporated entirely herein by reference.

As described in further detail below, 3D data acquisition methods canfurther include capabilities such as film thickness measurement, whitelight interferometry, Nomarski or differential interference contrast(DIC), and polarized light imaging.

FIG. 1A illustrates an exemplary illuminator 100 that can be used in a3D imaging and metrology system of the present invention. Illuminator100 contains two light sources 101 and 102. In one embodiment, lightsources 101 and 102 can include high brightness white color lightemitting diodes (LEDs). Other light sources such as halogen lamps, fibercoupled lights, lasers, and etc. can also be used.

Light sources 101 and 102 form two light paths as illustrated by thedot-dashed lines in the lower half of FIG. 1A. These two light pathsdefine the basic 3D imaging mode of operation of the system. Both lightpaths share several components, namely a first beam-splitter 103, anachromat doublet lens 105, a double convex lens 106, and a secondbeam-splitter 107A. Note that in other embodiments, lenses 105 and 106can be implemented with other types of lenses providing similar opticalfunctions. Light source 102 launches a second light path which includeslight source 102, a patterned article 104A or 104B, and the sharedcomponents mentioned earlier. Patterned articles 104A and 104B aremounted on a linear slider 104. Although a linear slider is the simplestway in this case to mount patterned articles 104A and 104B, other typesof multi-position fixtures such as a circular turret and etc. can alsobe used and are within the scope of this invention.

Beam-splitter 107A is mounted on a linear slider 107 linked to a pulllever, as are two other beam-splitters 107B and 107C. In one embodiment,linear slider 107 can be implemented by a detent spring plunger thatstops at two predefined positions, i.e. with beam-splitter 107Apositioned to direct the beam or beam-splitters 107B/107C positioned todirect the beam. Thus, beam-splitters 107A, 107B, and 107C will not bein the path at the same time. The term “beam-splitter set”, as usedherein, refers to beam-splitter 107A individually or beam-splitters 107Band 107C in combination. As described in further detail below,beam-splitter 107A is positioned to direct the illuminating lightdownward to an objective and then direct the returning light reflectedfrom the sample to a camera. When beam-splitters 107B and 107C are inthe path, beam-splitter 107B directs illuminating light downward to theobjective and then directs the returning light reflected from the sampleto beam-splitter 107C, which has an orientation different than that ofbeam-splitter 107B. In this orientation, beam-splitter 107C can directpart of the returning beam to lens 110. In one embodiment, the diameterof beam-splitter 107A is 1.0 inch while that of beam-splitters 107B/107Cis 0.5 inches.

The optical components of illuminator 100 are typically mounted inside adark enclosure with two openings (not shown): a top opening and a bottomopening. The top opening can be directly above beam-splitter 107A, whilethe bottom opening can be directly below beam-splitter 107A. These twoopenings allow light from both light paths to interact with other systemcomponents that do not belong to illuminator 100. A multi-pin connector108 is linked to light sources 101 and 102 via electrical wires.

FIG. 1B illustrates an exemplary patterned article 104A or 104B. Thepatterned article can be a piece of glass, liquid crystal, orphotographic film (i.e. the substrate) with a two dimensional array ofevenly spaced opaque dots formed on one of its surfaces. Different typesof patterns such as a grid, diamonds, etc. can also be used. Indeed, anypattern will work as long as it satisfies the following conditions: (1)it has high contrast, (2) it is either regular or random, (3) it issemi-transparent, and (4) its minimum feature size matches samplingresolution of an imaging optical sensor used. The patterned surface ofthe patterned article is located at the effective focal plane of thelens group, i.e. lenses 105 and 106. As described in further detailbelow, the patterned article can be used in the illuminator to projectan image of the pattern onto the focal plane of an objective lens tocreate enough contrast such that 3D height information of a sample canbe obtained.

Notably, patterned articles 104A and 104B, which differ in patternpitch, can be selected to match specific optical sensor/coupling lenscombinations to achieve optimized imaging results. Depending on thecircumstances, linear slider 104 can position patterned article 104A orpatterned article 104B in the light path. Plug 109 can fill an open sloton one side of illuminator 100, such slot being reserved for componentsthat can provide Nomarski or DIC, polarized light, and phase-shiftinginterference imaging, all of which is discussed below.

A third light path, as illustrated by the dot-dashed lines in the upperhalf of FIG. 1A, can be used in a film thickness measurement mode ofoperation. In this mode, linear slider 107 is positioned so thatbeam-splitters 107B and 107C are in the illumination light path (i.e.instead of beam-splitter 107A). Light from light source 101 passesthrough beam-splitter 103, lenses 105 and 106, and is directed by beamsplitter 107B to travel downward into an objective lens (described inreference to FIG. 2A). The light hits the sample surface, reflects back,then goes through the objective lens, beam splitter 107B, andbeam-splitter 107C. Beam-splitter 107C then directs the lighthorizontally towards a focusing lens 110. After lens 110, the convergingbeam makes two 90-degree turns upon hitting mirrors 111 and 112, andfocuses near the exit of a connector 113. The focused light beam thenenters a fiber cable 114 and is collected by a spectrometer 120, whichis connected to fiber cable 114.

FIG. 1C shows an exemplary illuminator 100C configured for Nomarski andpolarized light imaging. In this configuration, patterned articles 104Aand 104B as well as a through-hole 104C are mounted on a linear slider104, and a polarizer assembly 115 replaces plug 109 (FIG. 1A).Through-hole position 104C is useful in certain Nomarski or polarizedlight imaging applications when maximum illumination light intensity isrequired for very small defect detection and light source 101 isinsufficient for such detection. Thus, using through-hole position 104Callows the light from both light sources 101 and 102 to be combined. Theorientation of a polarizer 115A can be adjusted via a thumb knob 115Bfor polarized light imaging applications. When used for Nomarskiimaging, however, polarizer 115A is set at a fixed orientation. Notethat for either Nomarski or polarized light imaging, linear slider 104could be positioned such that through-hole 104C is in the light path tomaximize the amount of illumination light on the sample (for example,for detection of tiny defects on a flat hard-drive disk surface).Otherwise, to acquire a 3D image, either pattern 104A or pattern 104Bcan be positioned in the path including light source 102].

FIG. 1D illustrates an exemplary illuminator 100D configured forquantitative differential interference contrast (q-DIC) and motorizedpolarized light imaging. In this configuration, plug 109 (FIG. 1A) isreplaced by an assembly 117 including polarizer 115A, which is mountedon a motorized rotator 115C, and a quarter wave plate 117A. Multi-pinconnector 108 (FIG. 1A) is replaced by multi-pin connector 108A, whichlinks to light sources 101 and 102, as well as to motorized rotator115C. The orientation of polarizer 115A can be adjustedquasi-continuously via motorized rotator 115C for polarized lightimaging applications. However, when used for q-DIC imaging (describedbelow), polarizer 115A can be stepped through five consecutive positionsequally spaced at 45° intervals or any other number/phase shift steps toextract phase information in q-DIC. Because the orientation of polarizer115A can be adjusted remotely, software can create specific recipes forboth polarized and q-DIC observation as well as imaging of particularsamples. For example, when an operator routinely inspects plasticsamples for residual stress and scratch defects, the inspector typicallyrotates the polarizer to one orientation to examine the stress and thenrotates the polarizer to another orientation to see the scratch defect.This process is tedious and error-prone.

In contrast, the operator can store the best polarizer orientationsettings for stress inspection and scratch detection in recipe files.The operator can then load these recipes preceding stress inspectionand/or scratch detection to ensure that the system is optimallyconfigured for those jobs. Advantageously, with these recipes, amotorized rotator, and software (executable by a processor or computer),stress or scratches can be detected without any human intervention. Thisilluminator configuration can greatly enhance the ease of use of thesystem, reduce operator-related errors, and improve data reliability.

FIG. 1E illustrates an exemplary illuminator 100E configured forphase-shifting interferometry (PSI) and vertical-scanning interferometry(VSI). In this configuration, plug 109 (FIG. 1A) is replaced by awavelength filter assembly 116, which includes a through-hole 116A and anarrow band filter 116B, and through-hole 104C (FIG. 1A) is replaced bya pin-hole 104D. By sliding filter assembly 116 in and out, eitherthrough-hole 116A or narrow band filter 116B can be positioned in theillumination light path, thereby enabling either VSI or PSI imaging(discussed below).

FIG. 2A illustrates an exemplary 3D imaging and metrology system 200 inaccordance with the present invention. System 200 includes illuminator100, which is shown in side view. Because illuminator 100 providesreflected illumination, it is called a reflected illuminator. To avoidcluttering, only those illuminator components that are visible from theoutside are shown in FIG. 2A.

Referring to both FIGS. 1A and 2A, a microscope objective lens 210 ismounted on a turret 205. Turret 205 is mounted directly below the bottomopening of illuminator 100. When light source 101 or 102 is turned onand beam-splitter 107A is in the illumination light path, the lens groupformed by lenses 105 and 106 projects an image of the light source ontothe entrance pupil of microscope objective lens 210, thereby ensuringuniform illumination on a sample 220. When light source 102 is turned onand beam-splitter 107A is in the illumination light path, the lens groupformed by lenses 105 and 106 projects an image of the pattern onpatterned article 104A or 104B onto the focal plane of objective lens210.

A positioning means 230 is provided to change the relative positionbetween sample 220 and objective lens 210. As a result, differentfeatures on the sample can be brought into focus of objective lens 210.A manual or motorized XY stage 225 can be incorporated into system 200to move sample 220 in the horizontal plane. In preferred embodiments,positioning means 230 is either a motorized Z stage or a motorized Zstage and a piezo-Z stage combination. Other embodiments may use otherways to vary the relative position between sample 220 and objective lens210. For example, objective lens 210 could be mounted on a piezoelectricactuator. In such an arrangement, sample 220 remains stationary whileobjective lens 210 moves up and down.

A tube lens 245 and optical sensor coupler 250 (together called a“coupler” in U.S. Pat. Nos. 7,729,049 and 7,944,609 as well as U.S.Published Applications 2010/0135573 and 2008/0291533) in conjunctionwith objective lens 210 yields an image of sample 220 on an opticalsensor 255. In preferred embodiments, optical sensor 255 is either acharge-coupled device (CCD) or a complementary metal-oxide-semiconductor(CMOS) camera. Plugs 215 and 235 can fill the open slots in tube lensadaptor 240 and turret 205, respectively. In other system embodiment,described below, these slots can be used for Nomarski and polarizedlight imaging. A processor 260 can be connected to system 200 (in someembodiments, processor 260 can form part of the system). Processor 260can be used to control positioning means 230, illuminator 100, aspectrometer (not shown), and optical sensor 255. In addition, processor260 can analyze data and create a 3-D image of the sample. In oneembodiment, processor 260 is a personal computer.

In the film thickness measurement mode, described in reference to FIG.1A, broadband surface reflectance data is collected for sample 220 aswell as for a known standard, such as a polished silicon wafer. Usingthis data, processor 260 can analyze the reflection spectrum provided byspectrometer 120 and calculate a film thickness value and optical indexof refraction of a thin film (if present) on sample 220.

This film thickness measurement is based on a broadbandspectrophotometry method. There are several ways to implement thismethod. One exemplary method is disclosed in U.S. Pat. No. 7,248,364,which issued to Hebert on Jul. 24, 2007. Another method is disclosed byFilmetrics, Inc. in their “F40 Thin-Film Measurement Systems” brochure.Note that although, the F40 system also uses microscope optics toachieve a small measurement spot size, reflected light for theirspectrometer is collected at the image light path, i.e. at a locationequivalent to between tube lens adaptor 240 and optical sensor coupler250 (FIG. 2A). Moreover, the camera coupler of the F40 system is acustom-made part that contains a 45-degree beam splitter to direct partof the return light towards a camera, which is mounted horizontally toone side of the camera coupler body.

In contrast, in system 200, reflected light for spectrometer 120 iscollected via a third optical path inside illuminator 100 that isindependent of the imaging path leading to optical sensor 255. Notably,optical sensor coupler 250 can be implemented with standard industrycomponents having a plurality of magnifications, all at relatively lowcost. Thus, compared to system 200, the F40 system suffers severalsignificant disadvantages. First, the custom-made camera coupler of theF40 system contains no lens and therefore is 1× in magnification,thereby limiting the imaging field of view to only one size for eachobjective lens. Second, if custom camera couplers with differentmagnifications are desired, the resulting cost will be very high. Third,even if the F40 system had expensive camera couplers custom made,swapping among them will be inconvenient because the collection fiber(i.e. the equivalent of fiber 114 in FIG. 1A) has to be removed firstand then re-installed after every swap. As a result, fiber alignment(i.e. lateral and focus adjustment) has to be performed for every swap.Fourth, the 45-degree beam splitter in the camera coupler of the F40system is in a converging beam path. As such, the beam splitter willintroduce aberration, which in turn will impact the shape of the focusedlight spot at the fiber entrance and reduce light collection efficiency.

FIG. 2B illustrates another exemplary 3D imaging and metrology system200B in accordance with the present invention. System 200B includesilluminator 100C (FIG. 1C), which is configured for Nomarski andpolarized light imaging. Referring to FIGS. 1C and 2B, a microscopeobjective lens 210 suitable for Nomarski and polarized light imaging ismounted on turret 205. Turret 205 is mounted directly below the bottomopening of illuminator 100C. When light source 101 or 102 is turned on,the lens group formed by lenses 105 and 106 projects an image of thelight source onto the entrance pupil of microscope objective lens 210,thereby ensuring uniform illumination on sample 220. When light source102 is turned on, the lens group formed by lenses 105 and 106 projectsan image of the pattern on patterned article 104A or 104B onto the focalplane of objective lens 210.

In Nomarski or DIC imaging mode, light from light source 101 and/or 102passes through polarizer 115A. The resulting linearly polarized lightwaves travel downward upon hitting beamsplitter 107A. These light wavesthen enter a Nomarski prism 275 (replacing plug 215 of FIG. 2A) locatedabove objective 210 in turret 205. In Nomarski prism 275, the lightwaves are sheared into two orthogonal components, i.e. the ordinary andextraordinary wavefronts. Objective 210 focuses these two wavefrontsonto the surface of sample 220 where their paths are altered due tomorphology and/or refractive indices change on the surface.

The reflected wavefronts are gathered by objective 210 and travel upwardthrough Nomarski prism 275 where they are recombined to eliminate shear.They pass through beamsplitter 107A and then encounter an analyzer 270,which is positioned with its transmission axis orthogonal to that ofpolarizer 115A. Wavefront components that are filtered by analyzer 270pass through tube lens 245 and optical sensor coupler 250, andsubsequently undergo interference in the image plane to form theso-called Nomarski image or differential interference contrast (DIC)image on optical sensor 255. If Nomarski prism 275 is adjusted formaximum extinction, then the resulted DIC image often has a darkbackground and exhibits very high sensitivity to slight phase gradientspresent in certain sample regions. A bias retardation can be introducedby shifting Nomarski prism 275 laterally. By doing so, wavefront pairsforming the background become out of phase relative to each other andthe degree of elliptical polarization is increased in the wavefrontsentering analyzer 270. As a result, the background intensity becomesbrighter and sample features increasingly resemble a pseudo 3D reliefimage with peaks and valleys depending on the phase gradientorientation. A good application of Normaski imaging is to discernfeatures with tiny depth or height relief. Exemplary features includesmall defects formed during the manufacturing process of magneticstorage disks.

In the polarized light imaging mode, Nomarski prism 275 can be pulledout of the light path, and the transmission axis of polarizer 115A canbe adjusted to maximize desired feature detection sensitivity via thumbknob 115B. Light from light source 101 and/or 102 passes throughpolarizer 115A. The resulting linearly polarized light waves traveldownward upon hitting beamsplitter 107A. Objective 210 focuses the lightwaves onto the surface of sample 220. If the sample surface containsboth polarization active and inactive materials, the reflectedwavefronts emanating from the polarization active region will have theirpolarization orientation altered while those from the polarizationinactive region will not. Polarization active materials possess certainproperties such as those found on non-linear metallurgical specimens.

The reflected wavefronts are gathered by objective 210 and travel upwardthrough beamsplitter 107A and then encounter analyzer 270 positionedwith the transmission axis nearly orthogonal to that of polarizer 115A.Wavefront components that are filtered by analyzer 270 pass through tubelens 245 and optical sensor coupler 250, and subsequently form apolarized light image on optical sensor 255. Because light reflectedfrom the polarization active region has a higher transmission ratethrough analyzer 270 than that of light from the polarization inactiveregion, one can easily discern features with different polarizationproperties in the image. An exemplary application for polarized lightimaging is locating small defects on a data storage disk pre-marked withmagnetic marking. Notably, under regular microscope imaging mode, themagnetic marking is invisible due to lack of image contrast. However,with polarized imaging, the magnetic marking is visible and can be usedto locate a particular defect. Once the defect is located, it can beanalyzed using a q-DIC method to ascertain whether the defect is aparticle or a pit, and to obtain its height or depth.

Note that in both Nomarski and polarized light imaging modes ofoperation, there are cases where more illumination light than either oneof the two light sources 101 and 102 can provide is needed. When thishappens, linear slider 104 can be adjusted such that through-hole 104Cis positioned in front of light source 102. As a result, light from bothlight sources 101 and 102 can be combined to illuminate sample 220,which results in maximum intensity for Nomarski or polarized lightimaging application.

Positioning means 230 is provided to change the relative positionbetween sample 220 and objective lens 210. As a result, differentfeatures on the sample can be brought into focus of objective lens 210.A manual or motorized XYθ stage 265 can be incorporated into system 200Bto move and rotate sample 220 around in a horizontal plane. In onepreferred embodiment, positioning means 230 is a motorized Z stage. Inother embodiment, other ways to vary the relative position betweensample 220 and objective lens 210 can be used. For example, objectivelens 210 could be mounted on a piezoelectric actuator. In such anarrangement, sample 220 remains stationary while objective lens 210moves up and down. Once again, processor 260 can be connected to system200B to control positioning means 230, illuminator 100C, spectrometer120, and optical sensor 255. In addition, processor 260 can analyze dataand create a 3-D image of sample 220. In one embodiment, processor 260includes a personal computer.

FIG. 2C illustrates another 3D imaging and metrology system 200C, whichincludes illuminator 100D configured for q-DIC and motorized polarizedlight imaging. Referring to FIGS. 1D and 2C, illuminator 100D canprovide Nomarski or DIC imaging as well as polarized light imaging (alldescribed above). As noted previously, illuminator 100D includesassembly 117, which in turn includes quarter wave plate 117A, polarizer115A, and motorized rotator 115C for polarizer 115A.

In operation, the fast axis of quarter wave plate 117A is fixed at a90-degree angle with respect to the transmission axis of analyzer 270.For Nomarski or DIC imaging mode, light from light source 101 and/or 102passes through polarizer 115A and quarter wave plate 117A. The resultinglight is typically elliptically polarized unless the transmission axisof polarizer 115A coincides with the fast axis of quarter wave plate117A, in which case, the light remains linearly polarized. Becauseelliptically polarized light represents a phase difference between theordinary and extraordinary wavefronts, bias is introduced to the systemwhen the wavefronts enter Nomarski prism 275 and become sheared.Therefore, the combination of polarizer 115A and quarter wave plate 117Aenables adjustment of the bias retardation that is usually achievedthrough laterally shifting Nomarski prism 275. Because polarizer 115A ismounted on a motorized rotator, the amount of bias retardation can beprecisely controlled, which is critical to q-DIC imaging.

FIG. 2D illustrates an exemplary 3D imaging and metrology system 200Dincluding illuminator 100E, which is configured for phase-shifting andvertical scanning interferometry. Referring to FIGS. 1E and 2D, system200D further includes a Michelson interference objective lens 280 and aMirau interference objective lens 285 mounted on turret 205. Michelsonand Mirau interference objectives are described, for example, in“Optical Shop Testing”, by Daniel Malarca, 2nd edition, John Wiley &Sons, Inc., 1992. In general, a Michelson interference objective is usedfor magnifications below 10× while a Mirau interference objective isused for 10× or higher magnifications. Turret 205 is mounted directlybelow the bottom opening of illuminator 100E. When light source 101 or102 is turned on, the lens group formed by lenses 105 and 106 projectsan image of the light source onto the entrance pupil of interferenceobjective lens 280 or 285, thereby ensuring uniform illumination onsample 220. When light source 102 is turned on, the lens group formed bylenses 105 and 106 projects an image of the pattern on patterned article104A or 104B onto the focal plane of interference objective lens 280 or285.

Positioning means 230 is provided to change the relative positionbetween sample 220 and interference objective lens 280 or 285. A manualor motorized XY plus Tip/Tilt stage combination 290 can be incorporatedinto system 200D to move sample 220 around in a horizontal plane and tolevel the sample surface. In one preferred embodiment, positioning means230 can include a motorized Z stage and a piezo-Z stage combination. Inother embodiments, other ways to vary the relative position betweensample 220 and interference objective lens 280 or 285 can be used. Forexample, objective lens 280 or 285 could be mounted on a piezoelectricactuator. In such an arrangement, sample 220 remains stationary whileobjective lens 280 or 285 moves up and down.

System 200D can select between two interference imaging modes: avertical scanning (VSI) mode and a phase-shifting interferometry (PSI)mode. In the VSI mode, the light from light source 101 passes throughbeam-splitter 103, lenses 105 and 106, and through-hole 116A on filterassembly 116. The light then travels downward upon hitting beam-splitter107A toward objective 280 or 285. Objective 280 or 285 splits light intotwo wavefronts, wherein one wavefront travels towards the surface ofsample 220 while the other wavefront travels sideways towards thesurface of a reference mirror inside objective 280 or undergoes multiplereflection between two parallel plates inside objective 285. Thereflected wavefronts from both the sample and reference surfaces aregathered by objective 280 or 285 and travel upward through beam-splitter107A, tube lens 245, optical sensor coupler 250, and subsequentlyundergo interference in the image plane to form an interference image onoptical sensor 255. To acquire data, positioning means 230 moves sample220 in the Z direction while optical sensor 255 captures interferograms.

The PSI mode differs from that of VSI in two aspects. First, filter 116Bis placed in the illumination light path to turn white light into a verynarrow band illumination light. Second, during data acquisition, thepiezo-Z within positioning means 230 makes four phase shift moves (e.g.0, π/2, π, and 3π/2) while optical sensor 255 captures fourinterferograms. In general, PSI is used to measure surfaces flat towithin one wavelength while VSI is used to measure surfaces with largerZ variations.

Prior to a PSI or VSI scan, the relative tilt between a sample surfaceand the interferometer system has to be adjusted. U.S. Pat. No.7,102,761, issued to De Lega on Sep. 5, 2006, discloses three ways tocarry out this type of alignment. In a first technique, the referencepath is adjusted so that a minimum number of interference fringes isvisible across the sample surface. In a second technique, a preliminaryoptical path difference (OPD) scan is performed. A least-square fit of aplane surface through the measure data can then calculates the amount oftip and tilt value that needs to be adjusted. In a third technique, anexternal autocollimator telescope measures the test object orientationon a fixture that it then places in front of the interferometer.

In practice, each of these three alignment methods has limitations or isnot user friendly. For example, the first technique is not user friendlyto a novice user because it is not easy to find interference fringes inthe first place. In addition, it is not intuitive to adjust the tip/tiltknobs to minimize the number of fringes. The second technique, althoughautomatic, requires expensive piezo-electric transducer drivenmechanisms to perform the technique. The third technique, which relieson an external measurement station, assumes that a “fixture” isrepeatable so that sample surface tilt at the pre-measurement stationand at the interferometer correlate. In one embodiment, the fixture canbe sample holder that can be placed on an external autocollimator tohave the sample surface leveling adjusted. However, this assumption maybe incorrect. Finally, when the amount of tilt is very large,interference fringes become so narrow that they are invisible. As such,the first and second techniques may fail to work. The third techniquemay also fail to work because the autocollimator has a small angularmeasurement range.

In contrast, system 200D permits new tip/tilt alignment procedures thatovercome these limitations. FIG. 3A illustrates an exemplary tip/tiltalignment technique 300. Step 301 can turn on light source 102. Step 302can reposition linear slider 104 so that pin-hole 107D is in theillumination light path. Step 303 can rotate turret 205 so that an emptyslot (i.e. one without an objective mounted) is in the imaging path,thereby transforming system 200D into an autocollimator.

If the amount of sample tilt is small, as determined by step 304, thenthe image of a bright pin-hole will appear in an otherwise pitch darkfield of view. For example, in one embodiment, the software of processor260 can overlay a pre-defined circle on the image screen. At this point,step 305 can adjust the tip/tilt mechanism of stage combination 290 sothat the pin-hole image coincides with the pre-defined circle, therebycompleting alignment.

If the amount of sample tilt is so large that the image of the pin-holefalls outside of the field of view, then either step 306 or step 308 canbe used to bring the image of the pin-hole into the field of view. Step306 performs a one-pass 3D imaging acquisition process disclosed incommonly assigned U.S. Pat. No. 7,944,609 to acquire a 3D image of thetilted surface. At that point, the software used in the process canautomatically indicate how much adjustment is needed and in whichdirection in order to level the surface. Step 307 can make the suggestedadjustment, thereby ensuring that the pin-hole image is in the field ofview. At that point, technique 300 can return to step 302 to finish thealignment process.

Alternatively, step 308 can adjust the tip/tilt mechanism with a userwatching for the pin-hole image to appear within the field of view. Notethat searching for a bright pin-hole image in a dark background is mucheasier than hunting for elusive interference fringes. Once the pin-holeimage is within the field of view, technique 300 once again returns tostep 302 to finish the alignment process.

Notably, tip/tilt alignment technique 300 works for any amount of sampletilt. This technique is intuitive and easy to follow because visualfeedback can be relied upon to move a pin-hole image to a pre-setlocation on the screen. The precision autocollimator is built intosystem 200D, so there is no uncertainty in the alignment result.

In commonly assigned U.S. Pat. Nos. 7,729,049 and 7,944,609 andco-pending U.S. Published Applications 20100135573 and 20080291533(collectively referenced herein as Zeta IP), both one-pass and two-pass3D image acquisition processes are disclosed. Any of these processes canbe applied to the system embodiments of FIGS. 2A-2D. FIG. 6 illustratesan exemplary one-pass 3D image acquisition process 600.

For example, after set-up (steps 601-605) of one-pass 3D imageacquisition process 600 in system 200 (FIG. 2A), positioning means 230can move sample 220 from a pre-determined start position away fromobjective lens 210 through a set of pre-determined steps. At each Zstep, processor 260 turns light source 102 on and light source 101 off(hereinafter referred to as Pattern ON). As a result, an image ofpatterned article 104A or 104B is projected onto the focal plane ofobjective lens 210, and optical sensor 255 captures and saves a firstimage of the sample. Then processor 260 turns light source 101 on andlight source 102 off (hereinafter referred to as Pattern OFF), andoptical sensor 255 captures and saves a second image of the sample (step606). This process repeats itself until all the steps have been taken(steps 607-609). When done, processor 260 analyzes the first and secondimage set to create a 3D image. Specifically, the image with the patterncan be used to generate a depth profile, whereas the image without thepattern can be used to generate an image intensity. Notably, thisone-pass 3D image acquisition process can be further improved asdescribed below.

First, it is unnecessary to take two images, one with Pattern ON andanother with Pattern OFF, at each Z step. For example, in oneembodiment, one Pattern OFF image can be captured for every few PatternON images without impacting the final 3D image quality. The impact isminimal because the Z information is derived from Pattern ON imagesonly. Because the maximum number of Z steps for each 3D scan is limitedby available computer memory, skipping certain number of Pattern OFFimages allows the Z step limit to be increased, thereby improving the Zresolution of a 3D image.

Second, allocating available Z steps to the entire Z scan range may beunnecessary. For example, referring to FIG. 3B, if a user wants detailedtexture information on the top, middle, and bottom surfaces of a sampleas well as accurate measurement on top-to-middle and top-to-bottom stepheights, then available Z steps can be allocated into three Z regions:between levels 331/332, 333/334, and 335/336, while skipping regionsbetween levels 332/333 and 334/335.

Third, covering a Z scan range in equal step size may be unnecessary.Suppose a user cares much more about the middle surface than the othertwo surfaces in FIG. 3B. In this case, the region between levels 333 and334 can be scanned in much finer step sizes than that for regionsbetween levels 331/332 and 335/336.

Fourth, if positioning means 230 is a motorized Z stage with a leadscrew actuator, then measurement accuracy is often affected by aphenomenon called stage drooping. Drooping occurs after the Z stagemakes a large move. For example, in one embodiment, prior to a 3D scan,the Z stage is moved up to a start position and scanned downward atsmall steps. Because the Z stage is slowly drooping down due to gravity,the effective Z step size for a downward scan is slightly larger thanthe indicated step size. As a result, the measured value on a standardstep height will be slightly lower than the real value for a downwardscan. In another embodiment, prior to a 3D scan, the Z stage is moveddown to a start position and scanned upward at small steps. Because theZ stage is slowly drooping down due to gravity, the effective Z stepsize for an upward scan is slightly smaller than the indicated stepsize. As a result, the measured value on a standard step height will beslightly higher than the real value for an upward scan. To reduce theimpact of drooping on measurement accuracy, an up-down scan procedurecan be used. For example, for every 3D image acquisition, an entire Zrange with both a downward scan and an upward scan can be performed. Theresults from the up and down scans can then be averaged to yield thestep height value.

The 3D image acquisition techniques described above and those describedin Zeta IP can also be applied to systems 200B (FIG. 2B) and 200C (FIG.2C). Note that Nomarski imaging provides an additional vehicle forobtaining 3D information. In general, a Nomarski system can becharacterized as a shear interferometer. In this case, the Nomarskiimage intensity corresponds very closely to the first derivative of theoptical path difference of the sample surface structure. With specialdata analysis algorithms, for example those described by E. B. VanMunster et. al., in “Journal of Microscopy”, vol. 188, Pt. 2, November1997, a 3D image of certain type of sample surfaces can be created froma single Nomarski image without any Z scanning. This type of imagingtechnique can be called a “limited” q-DIC. FIG. 4 shows a limited q-DICimage of an II-VI compound semiconductor wafer surface acquired on animplemented system 200B (FIG. 2B). In this actual surface, the measuredsurface roughness value Ra is 1.4 nm.

For a more general q-DIC that works on all types of sample surfaces,q-DIC system 200C (FIG. 2C) can be used. In general, the DIC imageintensity can be described by the following formula (see also E. B. VanMunster et. al., in “Journal of Microscopy”, vol. 188, Pt. 2, November1997).

I(x,y)=I _(min(x,y))+½[I _(max(x,y)) −I _(min(x,y))]×[1+cos(Δφ(x,y)+δ)]

where I(x,y) is the intensity measured at location (x,y); I_(min(x,y))is the residual intensity at destructive interference; I_(max(x,y)) isthe intensity measured at constructive interference; δ is the additionalphase shift, or bias; and Δφ(x,y) is the difference between the phase atlocation (x−½Δx, y−½Δy) and the phase at location (x+½Δx, y+½Δy), where(Δx,Δy) is the lateral shear introduced by the Nomarski prism.

Based on the intensity equation above, Δφ(x,y) can be obtained byacquiring five consecutive images, each at 90° phase shift from itsnearest neighbor, and using the Hariharan algorithm (see, for example,Daniel Malacara, “Optical Shop Testing”, 2^(nd) edition, John Wiley &Sons, Inc., 1992) to extract Δφ(x,y) as follows:

${{\Delta\phi}\left( {x,y} \right)} = {\tan^{- 1}\left\lbrack \frac{2\left( {I_{2} - I_{4}} \right)}{{2I_{3}} - I_{5} - I_{1}} \right\rbrack}$

Once Δφ(x,y) is obtained, the phase Ψ(x,y) or 3D surface topography canbe recovered by integrating Δφ(x,y) in the shear direction.

In practice, this q-DIC technique can be used to image sample surfacesflat to within 70 nm. Rougher surfaces can be measured with the 3D imageacquisition technique of Zeta IP as well as the four improvements toFIG. 6 described above. The combination of 3D scanning capability andthat of q-DIC systems (FIGS. 2A, 2B, and 2C) create a powerful metrologysystem that can measure surface features ranging from millimeters tosub-nanometers.

The 3D image acquisition process using VSI or PSI in system 200D (FIG.2D) is somewhat different. For VSI, only light source 101 is used, i.e.light source 102 is turned off. Filter slider 116 is positioned suchthat through-hole 116A is in the light path. Processor 260 commandspositioning means 230 to move sample 220 to a pre-determined startposition and then initiate a Z scan through a set of pre-determinedsteps. At each Z step, optical sensor 255 captures and saves an image ofthe sample interferogram. This process repeats itself until all thesteps have been taken. When done, processor 260 analyzes theinterferograms corresponding to each Z step using well-knownpeak-sensing algorithm (see, for example, James C. Wyant et. al., SPIEvol. 2782, 1996) to obtain surface height information. Because a Z movesignificantly smaller than the depth of focus of interference objectiveshifts the location of white light interference fringes, a true-color 3Dimage of the sample surface can be created from the VSI scan data. The3D image construction process is as follow: for every XY pixel location,corresponding interferogram data of all the Z steps can be sorted usinga peak-sensing algorithm to find Z_(peak), i.e. the surface position. Atthis point, the color values C_(peak) corresponding to this XY pixel canbe extracted from an image taken at predetermined Z steps away from theZ_(peak) position, knowing that interference fringes have passed thepixel at that point. The true color 3D image can then be reconstructedwith data set (x, y, Z_(peak), C_(peak)).

For PSI, only light source 101 is used, i.e. light source 102 is turnedoff. Filter slider 116 is then positioned such that narrow-band filter116B is in the light path. Processor 260 commands the piezo-Z withinpositioning means 230 to make four phase moves (e.g. corresponding tophase shifts of 0, π/2, π, and 3π/2) while optical sensor 255 capturesfour interferograms. When done, processor 260 analyzes theinterferograms corresponding to each Z step using the following formula(see also Daniel Malacara, Optical Shop Testing, 2^(nd) edition, JohnWiley & Sons, Inc., 1992) to obtain the wavefront phase difference φ(x,y) and the surface height information H(x,y) for every pixel (x, y).

${\phi \left( {x,y} \right)} = {\tan^{- 1}\left\{ \left\lbrack \frac{{I_{4}\left( {x,y} \right)} - {I_{2}\left( {x,y} \right)}}{{I_{1}\left( {x,y} \right)} - {I_{3}\left( {x,y} \right)}} \right\rbrack \right\}}$

and H(x,y)=λ φ(x, y)/2πwhere I₁ (x, y), I₂(x, y), I₃(x, y), and I₄(x, y) are intensity valuesfor pixel (x, y) at each Z step.

Often times, an image acquired with a large field of view setting willhave some degree of intensity non-uniformity, field curvature, andresidual slope due to imperfections in the optical components of asystem. There are several methods that can be employed to reduce theseundesirable effects.

In the first method, the software can subtract each image being acquiredwith a reference image. A reference image is an image taken with thesame objective, camera and coupler combination of a flat surface devoidof any detail. The resulting difference image can then have theimperfections removed.

A second method involves generating a mathematical representation of thegradual changes in image brightness from the center of the image towardthe periphery. Because most intensity non-uniformity effects areradially symmetric, the mathematical model can specify brightnesscorrection that can be applied at each radial distance from the centerof the image.

For universal slope in the image due to sample surface tilt, thebuilt-in autocollimator can be used to level the sample surface if theamount of tilt is small and the 3D imaging method disclosed in Zeta IPif the tilt is large. This method can be applied to both interferenceimaging methods as well as the q-DIC imaging method. The levelingprocess can be automated by adding a motorized tip and tilt stage 510 tosystems 200C and 200D, as shown in FIGS. 5A and 5B, respectively.

Advantageously, system 200 (FIG. 2A) can be easily transformed intosystem 200B (FIG. 2B) or system 200D (FIG. 2D) by positioning sliders,inserting filters, and swapping peripheral components (such asobjectives and sample stages). System 200 (FIG. 2A) can also be easilytransformed into system 200C (FIG. 2C) using a simple swap ofilluminators. In summary, the 3D imaging and metrology system of thepresent invention combines capabilities of 3D scanning imaging, VSI andPSI interferometry, film thickness measurement, and Nomarski andpolarized light imaging. Each of these functionalities is oftenaddressed with separate machines in the industry. In contrast, thepresent invention can integrate a set of these capabilities into acompact, low cost, and robust system package, thereby resulting in apowerful 3D imaging and metrology system that can benefit a wide varietyof industrial as well as R&D applications.

The embodiments described herein are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. As such, manymodifications and variations will be apparent. Accordingly, it isintended that the scope of the invention be defined by the followingClaims and their equivalents.

1. A method of 3D imaging or measuring a sample, the sample including aplurality of surface features with different vertical levels, the methodcomprising: varying a relative distance between the sample and anobjective lens at predetermined steps; at first predetermined steps:projecting an image of a patterned article onto a focal plane of theobjective lens; capturing a first image with a pattern associated withthe patterned article and the sample, and storing the first image in afirst image array; at second predetermined steps, wherein the secondpredetermined steps have a different number of steps than the firstpredetermined steps: capturing a second image of the sample without thepattern associated with the patterned article, and storing the secondimage in a second image array; and analyzing the first and second imagesto 3D image or measure the sample.
 2. The method of claim 1, wherein anumber of the second predetermined steps is less than that of the firstpredetermined steps.
 3. The method of claim 1, wherein the first andsecond predetermined steps are allocated to specific levels.
 4. Themethod of claim 3, wherein at least one of the first and secondpredetermined steps has finer step sizes at specific levels.
 5. Themethod of claim 1, wherein the first and second predetermined steps skippredetermined levels of the sample.
 6. The method of claim 1, wherein atleast one of the first and second predetermined steps has finer stepsizes at specific levels.
 7. A method of 3D imaging or measuring asample, the method comprising: varying a relative distance between thesample and an objective lens at predetermined steps during an upwardscan; at first predetermined steps of the upward scan: projecting animage of a patterned article onto a focal plane of the objective lens;capturing a first image with a pattern associated with the patternedarticle and the sample, and storing the first image in a first imagearray; at second predetermined steps of the upward scan: capturing asecond image of the sample without the pattern associated with thepatterned article, and storing the second image in a second image array;analyzing the first and second images to generate a first 3D image ofthe sample; varying a relative distance between the sample and theobjective lens at predetermined steps during a downward scan; at firstpredetermined steps of the downward scan: projecting an image of apatterned article onto a focal plane of the objective lens; capturing athird image with a pattern associated with the patterned article and thesample, and storing the third image in a third image array; at secondpredetermined steps of the downward scan: capturing a fourth image ofthe sample without the pattern associated with the patterned article,and storing the fourth image in a fourth image array; analyzing thethird and fourth images to generate a second 3D image of the sample; andanalyzing the first and second 3D images to determine drooping effectsand then provide a corrected 3D image and measurement results of thesample.
 8. A method of 3D imaging or measuring a sample, the methodcomprising: acquiring a 3D image of a flat reference surface devoid ofany features by varying a relative distance between the referencesurface and an objective lens at predetermined steps; at firstpredetermined steps: projecting an image of a patterned article onto afocal plane of the objective lens; capturing a first image with apattern associated with the patterned article and the reference surface,and storing the first image in a first image array; at secondpredetermined steps: capturing a second image of the reference surfacewithout the pattern associated with the patterned article, and storingthe second image in a second image array; analyzing the first and secondimages to generate a 3D image of the reference surface; acquiring a 3Dimage of the sample by varying a relative distance between the sampleand the objective lens at predetermined steps; at third predeterminedsteps: projecting an image of a patterned article onto a focal plane ofthe objective lens; capturing a third image with a pattern associatedwith the patterned article and the sample, and storing the third imagein a third image array; at fourth predetermined steps: capturing afourth image of the sample without the pattern associated with thepatterned article, and storing the fourth image in a fourth image array;analyzing the third and fourth images to generate a 3D image of thesample; and subtracting the 3D image of the sample with that of thereference surface to yield a final 3D image and measurement results ofthe sample.
 9. A method of repositioning a sample to minimize tilt, themethod comprising: turning on a light source of a 3D microscope, thelight source passing through a pin-hole; transforming the 3D microscopeinto an autocollimator; when an image of the pin-hole falls inside afield of view, adjusting a tip/tilt mechanism of a stage of the 3Dmicroscope so that the pin-hole image coincides with a pre-definedcircle on an otherwise pitch dark field of view, thereby completingalignment; when the image of the pin-hole falls outside the field ofview, performing a one-pass 3D imaging acquisition process andadjustment of the stage based on the process to bring the image of thepin-hole into the field of view, then adjusting the tip/tilt mechanismof the stage of the 3D microscope so that the pin-hole image coincideswith the pre-defined circle on the otherwise pitch dark field of view,thereby completing alignment.
 10. A method of repositioning a sample tominimize tilt, the method comprising: turning on a light source of a 3Dmicroscope, the light source passing through a pin-hole; transformingthe 3D microscope into an autocollimator; when an image of the pin-holefalls inside a field of view, adjusting a tip/tilt mechanism of a stageof the 3D microscope so that the pin-hole image coincides with apre-defined circle on an otherwise pitch dark field of view, therebycompleting alignment; when the image of the pin-hole falls outside thefield of view, coarse adjusting the tip/tilt mechanism while watchingfor the pin-hole image to appear within the field of view, and then fineadjusting the tip/tilt mechanism of the stage of the 3D microscope sothat the pin-hole image coincides with the pre-defined circle on theotherwise pitch dark field of view, thereby completing alignment.